Optical reflecting telescopes are telescopes which essentially use reflective optical elements (mirrors). An optical reflecting telescope is essentially made of a main mirror and a capture mirror that is mounted in the same tube. These mirrors are also designated as primary and secondary mirrors. Contrary to a lens arrangement of a spy glass, incident light is not refracted but reflected by the primary mirror. The tube or support structure is typically configured as a tubular or lattice structure wherein the primary mirror is arranged at one end and the secondary mirror is arranged at another end.
US 2011/0310502 (A1) discloses an optical space surveillance system including a primary mirror, a secondary mirror a support base and an optical measuring device.
U.S. Pat. No. 5,058,993 discloses an optical bench with a pair of end plates which are fixated at each other by a number of tubular connecting elements.
Optical reflecting telescopes can be implemented as a matter of principle either as “on-axis-optics” or “off-axis-optics”. It is an essential difference whether optical axes of the mirror surfaces coincide with their geometric axes (on-axis) or not (off-axis).
In both cases the primary and the secondary mirrors have to be positioned very precisely relative to each other so that on the one hand side their optical axes that are arranged either centric or outside of the mirror surfaces are oriented precisely coaxial in an ideal case and on the other hand side so that the set focal distance remains invariant under ail external conditions. In case of off-axis mirrors the roll angle to be adjusted has to be kept robust with respect to a longitudinal direction of the optical reflecting telescope over a long time period.
Deviations from a coaxial alignment would cause imaging errors or degrade spot quality and spot radius. Changes of the focal length would cause a change of the focusing plane. Furthermore large reflecting telescope pivot angles are very challenging for the mass and stiffness properties of the support structure in the gravity field of the earth in combination with high tracking accelerations wherein the support structure also has to comply with substantial thermal requirements. Off-axis-optics make it even more difficult to implement a thermally and mechanically robust structure due to an asymmetrical arrangement of optical elements and due to the additional degree of freedom that has to be controlled.